The most popular power source for automotive applications is an internal combustion engine connected to a mechanical drive train which in turn rotates at least one wheel to drive the automobile. However, state and federal automotive emission laws are becoming increasingly more difficult to meet using current internal combustion engines powered by hydrocarbon fuels which emit large quantities of carbon dioxide, carbon monoxide, and various nitrogen oxides as by-products. Additionally, even the most efficient internal combustion engines are not very efficient, having a maximum efficiency of approximately 35% or less. The efficiency of an internal combustion engine increases as the energy output increases. During urban driving cycles where the required power output is the lowest, the efficiency is even lower.
As an alternative, electric vehicles were developed with the electric energy stored in large battery packs that replace the internal combustion engine and powered the automobile. The stored energy drives at least one electric motor which in turn rotates at least one drive wheel. However, there are still many drawbacks with the expensive battery packs such as long recharge periods, minimal driving distances before recharging, lack of power for passing and climbing hills, and excessively large and heavy battery packs.
Hybrid vehicles have been designed wherein for example, the automobile is continuously powered by a relatively low-powered, fuel-burning power source such as turbogenerator which can be run near or at peak efficiency. When extra power is required, for example, during acceleration or hill climbing, the surge power is provided by a storage device which stores energy during steady state driving conditions such as driving at a constant speed on a level road. Prior art storage devices include flywheels, pressurized fluid accumulators, batteries, and capacitors. The fundamental problem with these systems is that maximum efficiency is still limited by the fuel burning combustion engine whether it be internal combustion, spark ignited, diesel or a gas turbine.
A relatively new technology proposed for powering automobiles is the use of fuel cells to provide electric power. A fuel cell creates electricity through a chemical reaction wherein a hydrocarbon fuel oxidizes and forms carbon dioxide and water as by-products. Fuel cells have an efficiency of approximately 70-80%, much more efficient than internal combustion engines at the relatively low power requirements of urban driving. However, for a fuel cell to provide all the necessary power to an automobile to climb hills, drive at highway speeds, and supply passing power, the fuel cell would be too large, expensive, and heavy for an automobile. See for example ASME 97-GT-340, Parametric Study of Fuel Cell and Gas Turbine Combined Cycle Performance by Stephenson, Dawn and Ritchey, Ian.